This invention relates generally to the field of cell biology and more specifically to cell adhesion systems.
Multicellular organisms, such as man, have some 10.sup.14 cells which may be divided into a minimum of fifty types, such as blood cells and nerve cells, etc. During the course of growth and development, cells adhere to other cells, or to extra-cellular materials, in specific and orderly ways. Such cell adhesion appears to be of importance in mediating patterns of cellular growth, migration and differentiation, whereby cells develop specialized characteristics, so as to function as, for example, muscle cells or liver cells. Cell adhesion mechanisms also appear to be implicated in dedifferentiation, notably where cells lose their specialized forms and become metastasizing cancer cells.
The adhesion of cells to other cells or extracellular substrates appears to be at least a two step process: a cell must first recognize an appropriate site for attachment and thence selectively attach to it. Certain molecules have been identified which promote the attachment of cells to a solid substrate. Among these are fibronectin and vitronectin, two glycoproteins which are found in human plasma and serum. Unequal concentrations of specific adhesion promoting proteins are present in various tissues, indicating that their functions may be somewhat different.
Adhesive proteins can be used, for example, to coat the surfaces of containers used for growing cells in tissue culture so as to promote the attachment of such cells. Conversely, by suspending such molecules in the culture medium, the cells may be discouraged from attaching to the substrate. Because various types of cells will only grow effectively when attached to a solid substrate, the regulation of attachment is important in enabling the culturing of cells.
To gain further insight into the mechanisms of cell adhesion so as to permit manipulation and control, an understanding of the recognition sites on the cells themselves is necessary. Recently, a cell surface receptor specific to fibronectin has been isolated and described (Pytela, et al., Cell 40:191, (1985 )). This fibronectin receptor appears to specifically recognize and bind a portion of the fibronectin molecule which is composed of the amino acid sequence arginine-glycine-aspartic acid (Arg-Gly-Asp). Upon introducing fibronectin receptors to a solid substrate to which is attached fibronectin, the receptors will selectively bind the protein and can thus be manipulated.
Because of the critical role of cell surface receptors in regulating cell growth and differentiation both in vivo and in vitro, there exists a need for the isolation and purification of additional cell surface receptors. Specifically, there is a need for cell surface receptors which may be specific to certain cell types, or exist in greater porportion in certain cell types, than the previously recognized fibronectin receptors. Further there exists a need to identify cell surface receptors which can be selectively manipulated with small and easily obtained molecules. Moreover, there exists a need for a method to protect the functional integrity of cell surface receptors during such manipulations. The present invention satisfies these needs and provides additional advantages as well.